For nearly two decades, there has been a growing need for gas sampling devices capable of applications beyond the more typical uses, as for example achieving law enforcement objectives (e.g., breathalyzers) and achieving medical objectives (e.g., patient breathing assist). Indeed, since 1970 when OSHA was established, there has been an increased awareness of the need to continuously monitor conditions in the workplace to assure compliance with Federal and State regulations.
Since breath is the only biological fluid that may be obtained noninvasively and on demand, it is currently the matrix of choice for a number of applications as for example in law enforcement and medical evaluation such as breathanalyzers and patient breathing assist. These uses generally rely on the fact that the concentration of the analyte of interest is in very high concentrations such as ethanol or carbon dioxide and can be analyzed with instrumentation that does not require separation of the analyte from other interferents.
Initial attempts at collecting exhaled breath samples for analyses of volatile substance content involved the use of two types of apparatus, namely the glass sample tube and the gas sampling bag. The glass sample tube permitted only a limited sample volume to be collected, and its use was short-lived. On the other hand, the gas sampling bag enjoyed a far longer usefulness for this purpose. Nevertheless, this apparatus has its shortcomings as well, and for those reasons its use also is inherently limited. Most significant among the objections is that in most circumstances the bag becomes bulky after sample collection and must be almost immediately transferred to a laboratory in order that desired analyses can be performed.
Furthermore concentration of a gas component using an absorbent is generally not feasible when using such gas collection containers and, therefore measuring an analyte in large volumes of exhaled breath that are contributed over a long period of time is not practical.
For the purposes of this art, two different breath samples can be taken, namely a "mixed" and an "end" or alveolar breath sample. A solvent in the deep lung or alveolar region of the lung is in intimate contact with solvent in the bloodstream. If a sample of solvent in the deep lung air is obtained that sample will be referred to as an alveolar or end-expired sample. As the solvent is exhaled, the sample becomes diluted with air in the upper respiratory track and is known as a mixed-expired sample. Generally an alveolar sample is regarded as being indicative of bloodstream solvent concentrations since that sample is in intimate contact with solvent in the blood stream. The manual technique for end-expired sampling requires the subject to hold his breath for about 20 seconds then to exhale, discarding the first 30-50% of the sample; and finally collecting the end-expired portion of the sample with the sampling device. There are also automated techniques for sampling end-expired air.
The concentrations of solvents in an exhaled breath sample are normally very low. Therefore, it has been found necessary to have the analytes in the bag sufficiently concentrated on an appropriate sorbent prior to analysis. In addition, if the analytes are stored in the bag for extended periods, severe losses of analyte may occur by absorption of the analyte into the bag wall or permeation of the analyte through the bag wall. In using the gas sampling bag, it has become apparent that concentration of the analytes on solid sorbent material is generally not feasible in the field. The only technique for concentrating the contents of the bag is via indirect means. The sample must first be trapped in the bag. A solid sorbent sampler is then connected at one end to the bag and the other end to a pump. A known volume of air in the bag is then sampled. Thus outside of the laboratory, neither the gas sampling bag nor the glass sample tube has been found to facilitate either direct concentration to volatile analytes in the samples taken or storage of the taken samples for extended periods of time.
Subsequently, other devices have been developed for sampling volumes of exhaled breath. For example, Boehringer et al. U.S. Pat. No. 4,046,014 discloses a charcoal tube sampler device for sampling respiratory gases in alveolar air. Another sampling device, which employs changes in pressure or flow rate in a main gas flow tube to initiate the sampling process as well as to terminate it, is disclosed in U.S. Pat. No. 4,297,871. Still another gas sampling device, disclosed in Ryan et al U.S. Pat. No. 3,858,593, incorporates a cylindrical alveolar gas trapping device having check valves at opposite ends which are openable upon application of exhalation pressure, and a side wall valved access tube for selective removal of the trapped gas from within the cylinder to a gas analyzer. Each of these subsequently developed devices also suffer disadvantages which make them undesirable for use. In particular, there is no provision for continuous mixed-expired sampling or filtering of inhalation air, and no provision for storing the collected gas sample for analysis at a subsequent time.